1. Field of the Invention
The present invention relates to a variable optical gain equalizer that converts an input light beam into an output light beam having a desired spectrum, and to a variable branching ratio beam splitter that is applied to the variable optical gain equalizer, etc., for branching the input light beam into two light beams at an arbitrary power ratio. These are especially useful techniques in, e.g., switching network or long-distance communication in optical communications.
2. Description of the Related Arts
In optical communication system using optical fibers as transmission lines, WDM (Wavelength Division Multiplexing) was developed as techniques for increasing the transmission capacity. This is an optical communication method in which a multiplicity of light signals having different wavelengths are transmitted through a single optical fiber. 8-32 waves wavelength multiplexing has hitherto been attempted, but recently more multiple wavelength multiplexing is also being studied.
Meanwhile, in order to render long-distance optical transmission feasible, the optical communication system having the transmission lines in the form of the optical fibers incorporates optical amplifiers to amplify signal light and thereby compensate for loss which may occur in the optical fibers, etc. In case of incorporating the optical amplifiers into the optical fiber transmission lines in the WDM optical communication system, however, there is a need to equalize the gain, due to the wavelength characteristics of amplification gains of the optical amplifiers. Means for gain equalization may be a single optical filter or a plurality of optical filters, having desired transmittance or desired insertion loss wavelength characteristics.
An optical equalizer constructed from a combination of a plurality of optical filters is capable of offsetting the transmitted light wavelength characteristics by the insertion loss waveform characteristics presented by the optical filters, in a given WDM signal light bandwidth. In contrast with the optical filter based loss wavelength characteristics being unvaried, however, the transmitted light wavelength characteristics vary by changes with time of other various optical elements used in the optical fiber transmission lines, making it difficult to offset the transmitted light wavelength characteristics by the optical filter wavelength characteristics. Switching of the optical fiber transmission lines also causes changes in the transmitted light wavelength characteristics, which changes could not be offset.
In order to solve the above problem, for example, Japanese Patent Laid-open Pub No. Hei11-224967 discloses a technique where a fixed optical gain equalizer is combined in series with a variable optical gain equalizer. The variable optical gain equalizer in this art is a combination of a variable optical attenuator and an optical amplifier. By utilizing the characteristics of the optical amplifier that depending on the light input level the state of operation varies and the optical amplifier gain wavelength characteristics also changes, this change is adjusted through the control of attenuation of the variable optical attenuator so as to accommodate the variance of the light input level.
However, this configuration is based on regulating the level of light input into the optical amplifiers by use of the fixed optical gain equalizer, and hence the fixed optical gain equalizer combined with the optical amplifier having specific characteristics has inevitably a restricted choice. Therefore, the adjustable light input level range is not too wide.
It is therefore an object of the present invention to provide a variable optical gain equalizer capable of freely converting an input light beam into an output light beam having a desired spectrum. This object includes providing a variable optical gain equalizer capable of easily performing gain equalization indispensable for the optical communication system, modifications resulting from path changes and modifications of changes with time of paths, as well as providing a variable optical gain equalizer of a structure suitable formodularization, allowing all optical elements to be integrated into a single package.
Another object of the present invention is to provide a variable branching ratio beam splitter applicable to, e.g., the variable optical gain equalizer of the present invention.
According to a first aspect of the present invention there is provided a variable optical gain equalizer comprising a variable branching ratio beam splitter having a variable polarization rotator, the beam splitter branching an input light beam into two light beams at an arbitrary power ratio, through control of the rotation angle of the polarization direction by the variable polarization rotator; an optical filter presenting different filter characteristics to the two branched output light beams; and a polarization coupler for polarization coupling the two light beams that have passed through the optical filter; wherein the rotation angle of the polarization direction is controlled by the variable polarization rotator so as to vary the spectra of the output light beams.
According to a second aspect of the present invention there is provided a variable optical gain equalizer comprising a variable branching ratio beam splitter for branching an input light beam into two light beams for output at an arbitrary ratio; an optical filter associated with at least one of branched outputs of the variable branching ratio beam splitter; and an optical coupler for coupling the branched two output light beams together; wherein the branching ratio of the variable branching ratio beam splitter is controlled so as to be able to adjust the spectra of light beams to finally be coupled by the optical coupler.
According to a third aspect of the present invention there is provided a variable optical gain equalizer comprising a beam multi-branching unit for branching an input light beam into three or more light beams; an optical filter associated with at least one of branched outputs of the beam multi-branching unit; and an optical star coupler for coupling together light beams branched by the beam multi-branching unit; wherein the beam multi-branching unit includes a variable branching ratio beam splitter disposed at one or more branched parts of the beam multi-branching unit, for branching an input light beam into two light beams for output at an arbitrary ratio; and wherein the branching ratio of the variable branching ratio beam splitter is controlled so as to be able to adjust the spectra of light beams to finally be coupled together by the optical star coupler.
According to a fourth aspect of the present invention there is provided a variable optical gain equalizer comprising a beam multi-branching unit including a plurality of variable branching ratio beam splitters which are in cascade connection, each variable branching ratio beam splitter branching an input beam into two light beams for output at an arbitrary ratio; an optical filter associated with at least one of branched outputs of the beam multi-branching unit; and an optical star coupler for coupling together light beams branched by the beam multi-branching unit; wherein the branching ratio of each of the plurality of variable branching ratio beam splitters is properly controlled so as to be able to adjust the spectra of light beams to finally be coupled together by the optical star coupler.
According to a fifth aspect of the present invention there is provided a variable optical gain equalizer comprising a variable branching ratio beam splitter which includes, at spaced apart locations in the mentioned order, a birefringent element for separation that separates light beams whose polarization directions are orthogonal to each other, input from the same optical path, into different optical paths; a birefringent element for optical path control that controls the optical path depending on the polarization direction; and a birefringent element for synthesis that synthesizes light beams whose polarization directions are orthogonal to each other, traveling along different optical paths; wherein the variable branching ratio beam splitter includes, between the birefringent element for separation and the birefringent element for optical path control, a polarization rotator for converting the polarization direction from orthogonal relationship to parallel relationship; and a variable polarization rotator for rotating arbitrary angles the polarization direction of light beams having parallel relationship, converted by the polarization rotator; wherein the variable branching ratio beam splitter includes, between the birefringent element for optical path control and the birefringent element for synthesis, a polarization controller for rotating 90 degrees the polarization direction of light beams traveling along two optical paths having specific diagonal relationship, the polarization controller keeping the polarization direction of light beams traveling along two optical paths having the other diagonal relationship; and wherein the angle of rotation of the polarization direction effected by the variable polarization rotator is controlled so as to branch an input light beam for output at a properly adjusted ratio.
According to a sixth aspect of the present invention there is provided a variable branching ratio beam splitter comprising, at spaced apart locations in the mentioned order, a birefringent element for separation that separates light beams whose polarization directions are orthogonal to each other, input from the same optical path, into different optical paths; a birefringent element for optical path control that controls the optical path depending on the polarization direction; and a birefringent element for synthesis that synthesizes light beams whose polarization directions are orthogonal to each other, traveling along different optical paths; wherein the variable branching ratio beam splitter comprises, between the birefringent element for separation and the birefringent element for optical path control: a polarization rotator for converting the polarization direction from orthogonal relationship to parallel relationship; and a variable polarization rotator for rotating arbitrary angles the polarization direction of light beams having parallel relationship, converted by the polarization rotator; wherein the variable branching ratio beam splitter comprises, between the birefringent element for optical path control and the birefringent element for synthesis: a polarization controller for rotating 90 degrees the polarization direction of light beams traveling along two optical paths having specific diagonal relationship, the polarization controller keeping the polarization direction of light beams traveling along two optical paths having the other diagonal relationship; and wherein the angle of rotation of the polarization direction effected by the variable polarization rotator is controlled so as to branch an input light beam for output at a properly adjusted ratio.